Referring to FIG. 12, the diameter φd of a hole to be drilled by a conventional drill bit is determined by the diameter φD of the drill bit, or more specifically, the diameter φd is defined by the cutting edge(s) of the drill bit. Since drill bits of a certain size can only be used to drill holes of a specific diameter, one who has drilling needs must prepare and store a large number of drill bits of various sizes. In addition, it is well known in the art that a twist drill bit as shown in FIG. 12 produces continuous and wide drilling chips during operation, especially when drilling mild steel, aluminum, aluminum alloys, or stainless steel. Such continuous chips do not break easily, tend to be stuck in the chip removing grooves, have high thermal conductivity, and generate high resistance, thus subjecting the main shaft of the controller, e.g., a machine tool, to a huge cutting load which is typically as high as 60% of the load capacity of the main shaft, and which has significant adverse effects on not only the main shaft, but also the rigidity, cutting precision, and service life of the machine tool. Continuous drilling, therefore, is impractical; the drilling process must be intermittent. Should the chips get stuck and make it impossible to keep on drilling, there is no other way than to suspend the drilling operation until the chips are removed.
FIG. 13 shows a conventional disposable drill bit whose disposable blades are provided with chip breaking grooves. When the material being drilled is soft, however, the chip breaking grooves may have problem breaking the drilling chips, and the chips may eventually get stuck and cause overheating. To cool the drill bit, it is typically required to pour a cutting fluid at the drill bit. But if the hole being drilled is so deep that the chips produced hinder the cutting fluid from flowing into the hole and reaching the cutting edges at the distal end of the drill bit, the temperature of the cutting edges will keep rising, and drilling must be temporarily stopped in order to remove the chips.
While drill bits with a water supplying feature at the center are commercially available, the equipment required for supplying water to the center of the drill bit is expensive, making up 30% or so of the cost of a machine tool. This explains why the penetration rate of such drill bits is only about 5%. Most drilling operations still have to be performed intermittently for chip removal and heat dissipation purposes and hence end up with low work efficiency and high processing cost.
Both drill bits described above are configured for continuous cutting and generate large, continuous chips which tend to get stuck and raise overheating issues. Moreover, a twist drill bit as well as a disposable drill bit has limitations on drilling depth, for the greater the drilling depth, the more difficult it is to discharge the chips produced. If the chips are trapped in the drill bit flutes, further drilling will be obstructed, and in order to carry on drilling, the drill bit must be drawn out to facilitate chip removal and heat dissipation.